Here is an article which describes one of the problems that can occur when utilizing custom claims providers:
Since custom claims providers are registered as a farm level feature, an exception in a single custom claim provider can cause the remainder of the claims providers to also break!
So what is the moral of this story? Thoroughly test your custom claims providers before deploying them into production! Make sure that all of the properties that are supposed to be true, are true. If you have not implemented a corresponding method for a property, make sure that property has been set to false.